Research On Operation Characteristic Of Ground Heat Exchanger For Ground Coupled Heat Pump System In Cold Climate

Ground coupled heat pump is a renewable energy technique for heating and cooling supply, which is well known for its energy conservation, environment protection. In recent years, with the improvement of the awareness of energy conservation and environment protection, increasing attention has been paid to ground coupled heat pump system. Because the initial soil temperature is low in cold climate, the soil freezing phenomenon is likely to occur when ground coupled heat pump system is utilized in winter. When the soil around ground heat exchanger is frozen, the thermal conductivity of soil will increase, and the latent heat released by phase transition of soil water is very large, which are benefit to enhance the heat transfer of ground heat exchanger. Whereas, only a few related researches has been done on this subject, and there is no clear conclusion of the exactly effect of soil freezing on enhancing the heat transfer of ground heat exchanger. And most of current heat transfer model of ground heat exchanger do not consider the nonlinear characteristic of heat transfer control equation when the soil is frozen. Although the performance of ground coupled heat pump in cold climate will decrease by the unbalance of cooling and heating load, the utilization of ground coupled heat pump in cold climate is still reasonable for some regions, such as regions far away from district heating plant and high standards are required for waste discharging. Therefore, the operation characteristic of ground heat exchanger for ground coupled heat pump system in cold climate will be studied in this thesis.Based on domestic and foreign achievements in the heat transfer model of ground heat exchanger, the heat transfer mechanism of ground heat exchanger in cold climate is analyzed. In view of the nonlinear characteristic of heat transfer equation with soil freezing, the pipe section of ground heat exchanger is transformed and the circular pipe is replaced with square pipe. The revised principle of pipe section transformation is provided and the phase transition of soil is solved by apparent heat capacity method. The heat transfer model takes the unsymmetrical heat transfer of inlet and outlet branch into account, and can be used to solve nonlinear heat conduction equation for multiple boreholes heat transfer problem. This model has laid foundation for simulating the performance of ground coupled heat pump system in cold climate.An experimental system is set up for low temperature heat extraction of ground heat exchanger. Based on the measuring results of continuous operation, the low temperature heat extraction characteristic of ground heat exchanger in cold climate is analyzed, and the heat transfer rate per unit depth of ground heat exchanger with continuous low temperature operation is obtained. Furthermore, the experimental results is used to validate the heat transfer model of ground heat exchanger with transformed pipe section.On the basis of the established heat transfer model ground heat exchanger which take soil freezing into account, the soil freezing characteristic around ground heat exchanger is studied through numerical simulation, some influence factors such as inlet fluid temperature, initial soil temperature, soil type and soil water content are analyzed. And the effect of soil freezing on heat transfer of ground heat exchanger is analyzed also. This part of work provides important theoretical support for accurately considering the effect of soil freezing on heat transfer of ground heat exchanger in actual project.With the heat transfer model for multiple boreholes, the factors which can affect operation characteristic of ground heat exchanger are analyzed, such as flow rate, pipe material, borehole depth, distance between two pipe of U tube, distance between boreholes, etc. Moreover, the heat transfer rate of ground heat exchanger in different position of borehole field and the heat transfer and soil temperature variation with long term single season operation are calculated and analyzed, and the recommended distance between boreholes is provided for ground heat exchanger in cold climate. This part is helpful to select reasonable design parameters of ground heat exchanger in cold climate.Taking a single building in HARBIN as an example, the whole year operation performance of ground coupled heat pump system in cold climate is studied. The performance in heating season is analyzed, including variation of inlet and outlet fluid temperature of ground heat exchanger, the heat extracted from soil and the heating supply performance of coefficient of the ground coupled heat pump system. In view of the low temperature of soil after heating season, comparison is made between ground coupled heat pump system supply cooling and ground sink direct cooling system, and the performance of both cooling supply mode is analyzed. According to the calculation of the heat extracted from soil and heat rejected into soil after whole year operation of ground coupled heat pump system, the imbalance rate of soil heat transfer is analyzed, which can provide reference for optimizing operation of ground coupled heat pump system in cold climate, and for taking technical measures to improve the unfavorable effect of soil heat transfer imbalance.